PHOTOCONTROLS
by Marvin Suggs

Photocontrols have changed very little since they were introduced over 40 years ago. They were invented to provide automatic, dusk to dawn control of outdoor lighting systems and fixtures. They have three basic components: (1) A cadnium sulfide photocell, (2) an electromagnetic or thermal relay and (3) a lightning/surge protector. They are manufactured by several companies, Ripley Sunswitch, Fisher Pierce and American Electric to name a few.

Photocontrols are very simple in their operation. The photocell varies the current flowing to the normally closed electromagnetic relay. When the photocell is exposed to sunlight, the resistance is zero, so the current flows to the relay energizing it. This in turn separates the relay contacts that controls the power to the light fixture. When the photocell is exposed to darkness the opposite happens. The resistance of the photocell goes to infinity, interupting the current to the relay, thus causing the spring loaded relay contacts to close. This in turn energizes the light fixture.

The photocontol has a three pronged plug on its base that twist-locks into a receptacle on the light fixture. Lights can be individually controlled or a group of lights can be controlled by a single photocontrol.

Most photocontrols in use today are the quick response electromagnetic type. The only drawback to these are that they are vulnerable to lightning strikes and artificial lights, that will momentarily turn off the light fixture. Photocontrols with a built in delayed response are used where these factors are a problem. This type of control has to be exposed to light for at least 3 seconds before the relay activates.

Some photocontrols that were made 30 years ago can still be seen in service. Very few things made today will last that long.

Thank you very much to Marvin Suggs for writing this article. Take a gander at Marvin's streetlighting/photocontrols page!